This invention relates to deposition of material onto a skirted substrate, and more particularly, relates to a method and apparatus for reducing the shadowing effect of the skirt on the thickness distribution of the deposited layers along the plane of the substrate.
U.S. Pat. No. 4,683,398 describes a cathode ray tube for projection color television having a multi-layer interference filter between the glass faceplate and the luminescent screen. Such a filter results, among other things, in significantly greater brightness of the luminescent output of the tube.
Such interference filters are typically composed of alternating layers of materials having a high and a low index of refraction, respectively. The layers are preferably formed by vapor deposition onto the inner surface of the glass faceplate.
Mass production of such filters is carried out in a vacuum chamber containing the source materials to be evaporated, means for heating the source materials, and a dome-shaped fixture adapted for holding a multiplicity of glass faceplates. The plates are arranged in rows, each row forming a circle around the dome, so that each plate is approximately equidistant with the other plates from the source materials.
The dome is rotated during deposition, not only to promote uniform distribution of the deposited material on the plates, but also to pass the plates behind one or more dodgers located between the source materials and the plates. These dodgers are designed to result in increasing thickness of the layers toward the edges of the plates, which has been shown to result in even greater increases in brightness than can be obtained for a uniform thickness distribution.
These glass faceplates often are fabricated with a peripheral upstanding wall, termed a "skirt". After deposition of the filter and formation of the luminescent screen on the inner surface of the faceplate, the edge of the skirt is then sealed to a matching edge of a glass funnel containing an electron gun in order to form the envelope of the projection tube.
The presence of the skirt causes a problem during filter deposition when there is a significant partial pressure of gas inside the vacuum chamber. Molecules evaporated from the source can be scattered by molecules in the gas, thereby becoming new sources. The skirt of the plate shadows a portion of the plate from these new sources of material, reducing the amount of material deposited in the shadowed region relative to the exposed areas of the faceplate and thus altering the thickness distribution of the filter layer from that desired.